A pulse oximeter estimates noninvasively the degree of oxygen saturation of the hemoglobin in the arterial blood. Modern instruments use optical techniques in conjunction with a noninvasive sensor to achieve the estimate. The sensor of the oximeter radiates a section of well perfused tissue with at least two wavelengths of light. The light contacts hemoglobin contained in red blood cells. A certain amount of light is absorbed by the hemoglobin. The amount of light absorbed depends on the wavelength of light and the level of hemoglobin oxygenation. By knowing the wavelength of light being used and the relative amount of light being absorbed, it is possible to estimate the blood oxygen saturation.
Most of the currently available oximeters using optical methods to determine blood oxygen saturation are based on transmission oximetry. These devices operate by transmitting light of at least two wavelengths through an appendage such as a finger or an earlobe. By comparing the characteristics of the light transmitted into one side of the appendage with that detected on the opposite side, it is possible to compute oxygen saturations. A major disadvantage of transmission oximetry is that it can only be used on portions of the body which are thin enough to allow passage of light. Another disadvantage is, if the patient goes into shock, the extremities are the first to loose blood flow. If blood flow is lost, the oximeter cannot compute oxygen saturation.
There has been considerable interest in recent years in the development of an oximeter which is capable of using reflected light of at least two wavelengths to measure blood oxygen saturation. The sensor of such a device has the light source positioned on the same side of the tissue as the detector. In this configuration, the detector receives only that light which is scattered back (reflected) to the detector. The specific layout of the light source and detector is critical in optimizing the signal-to-noise ratio and thus improving the accuracy of the device.
A reflectance oximeter is especially useful for measuring blood oxygen saturation in portions of the patient's body which are not well suited to transmission measurements. However, a major disadvantage with previous reflectance oximeters has been that the oximeter sensors failed to offer a technique in which the detection depth in the tissue from which the light is reflected could be controlled and optimized. Without a technique in which the detection depth for the sensor can be optimized, a less accurate indication of blood oxygen saturation may result. Reflected signals are weaker than transmissive signals, and the previous reflective sensors have failed to provide a technique to optimize the detection depth in the tissue from which the reflected signals are received by the optical detector. Particularly, the ratio of the pulsatile component (AC) and average (DC) is important to improve the measurement accuracy. Specific sensor design based on the theoretical model is necessary to optimize such performance. The work by Takatani et al, "Experimental and Clinical Evaluation of Non-Invasive Reflectance Pulse Oximeter Sensor", Journal of Clinical Monitoring (accepted for publication), revealed that the larger the spacing between the light source and detector, the larger the AC/DC ratio. However, the larger the separation distance, the smaller the absolute signal level. The actual measurement in tissue revealed that separation distance of 3 to 4 mm will be optimum from an instrument point of view. In addition to optimizing the distance between the light source and the detector, optimization in controlling the detection depth is needed to minimize large background DC levels due to the skin layer adjacent to the sensor surface. The detection depth should be focused deeper than the adjacent skin layer which is approximately 0.3 to 0.5 mm.
Thus, there remains a strong need in the art for a noninvasive reflectance oximeter sensor in which the detection depth in the tissue may be controlled so that a more accurate indication of blood oxygen saturation is achieved.